Laser Doping of Crystalline Semiconductors Using a Dopant-Containing Amorphous Silicon Stack for Dopant Source and Passivation

ABSTRACT

Techniques and structures for laser doping of crystalline semiconductors using a dopant-containing amorphous silicon stack for dopant source and passivation are provided. An example method includes forming a dopant-containing amorphous silicon layer stack on at least one portion of a surface of a crystalline semiconductor layer; and irradiating a selected area of the dopant-containing amorphous silicon layer stack, wherein the selected area of the dopant-containing amorphous silicon layer stack interacts with an upper portion of the underlying crystalline semiconductor layer to form a doped, conductive crystalline region, and each non-selected area of the dopant-containing amorphous silicon layer stack remains intact on the at least one portion of the surface of the crystalline semiconductor layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.13/646,120, filed Oct. 5, 2012, which is a continuation of U.S. patentapplication Ser. No. 13/645,926, filed Oct. 5, 2012, both of which areincorporated by reference herein.

FIELD OF THE INVENTION

Embodiments of the invention generally relate to electronic devices and,more particularly, to doping semiconductor solar cell devices.

BACKGROUND OF THE INVENTION

Challenges exist in obtaining solar cell back surface fields (bsfs) on alow-to-moderate thermal budget (for example, <400-800 C) in p-typesilicon (Si). Aluminum- (Al-)based bsfs (fabricated by >800 C alloyingof an Al-paste or metallic Al layer) can have the desired several-micronthickness, but can be difficult to form at temperatures below 800 C dueto the low solid solubility of Al in Si. While boron (B) dopants havemuch higher solid solubilities in Si, the long periods at hightemperatures (for example, 900-1050 C) required for sufficient Bdiffusion can degrade the bulk lifetime of the Si substrate and/or leadto dopant clustering in ways that can produce misfit dislocations.

Some back surface field functionality can be provided in heterojunctionwith intrinsic thin layer (HIT) cells with the use of intrinsicamorphous silicon (i-aSiH)/doped-aSiH stacks on Si substrates (forexample, p-Si(substrate)/i-aSiH/p-aSiH andn-Si(substrate)/i-aSiH/n-aSiH)), but these cells can be difficult tofabricate due to the narrow process window for providingi-aSiH/doped-aSiH stacks with aSiH layers thick enough to provide goodpassivation yet thin enough to provide sufficient tunneling current tothe back surface metallurgy.

Accordingly, a need exists for a low-temperature, easy-to-integratetechnique for forming B-doped back surface fields in p-type Si.

SUMMARY OF THE INVENTION

In one aspect of the invention, techniques for laser doping ofcrystalline semiconductors using a dopant-containing amorphous siliconstack for dopant source and passivation are provided. An exemplarymethod for forming at least one doped, conductive crystalline region ona surface of a crystalline semiconductor layer can include steps offorming a dopant-containing amorphous silicon layer stack on at leastone portion of a surface of a crystalline semiconductor layer, andirradiating a selected area of the dopant-containing amorphous siliconlayer stack, wherein the selected area of the dopant-containingamorphous silicon layer stack interacts with an upper portion of theunderlying crystalline semiconductor layer to form a doped, conductivecrystalline region, and each non-selected area of the dopant-containingamorphous silicon layer stack remains intact on the at least one portionof the surface of the crystalline semiconductor layer.

In another aspect of the present invention, techniques for forming adoped, conductive crystalline region on a surface of a crystallinesemiconductor layer can include steps of forming a dopant-containingamorphous silicon layer stack on at least one portion of a surface of acrystalline semiconductor layer, and irradiating the dopant containingamorphous silicon layer stack, wherein the dopant-containing amorphoussilicon layer stack interacts with an upper portion of the underlyingcrystalline semiconductor layer to form a blanket doped, conductivecrystalline region.

These and other objects, features and advantages of the presentinvention, particularly those relating to improved solar cell structuresand fabrication methods, will become apparent from the followingdetailed description of illustrative embodiments thereof, which is to beread in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A through FIG. 1C are cross-section view diagrams illustrating anexample process schematic for using a dopant-containing amorphoussilicon layer stack to form localized doped crystalline regions,according to an embodiment of the invention;

FIG. 2A through FIG. 2C are cross-section view diagrams illustrating anexample process schematic for using an overlayer-coateddopant-containing amorphous silicon layer stack to form localized dopedcrystalline regions, according to an embodiment of the invention;

FIG. 3A and FIG. 3B are cross-section view diagrams illustrating aconfiguration of overlayers that may be used in combination with adopant-containing amorphous silicon layer stack to form localized dopedcrystalline regions, according to an embodiment of the invention;

FIG. 4A and FIG. 4B are cross-section view diagrams illustrating anadditional configuration of overlayers that may be used in combinationwith a dopant-containing amorphous silicon layer stack to form localizeddoped crystalline regions, according to an embodiment of the invention;

FIG. 5A through FIG. 5G are cross-section view diagrams illustratingexamples of different solar cell structures in which localizedcrystalline doped regions may be incorporated as localized bsfs,according to an embodiment of the invention;

FIG. 6A and FIG. 6B are cross-section view diagrams illustrating a firstexample process schematic for using a dopant-containing amorphoussilicon layer stack to form a blanket doped crystalline region,according to an embodiment of the invention;

FIG. 7A through FIG. 7E are cross-section view diagrams illustrating asecond example process schematic for using a dopant-containing amorphoussilicon layer stack to form a blanket doped crystalline region,according to an embodiment of the invention;

FIG. 8A through FIG. 8C are cross-section view diagrams illustratingexamples of different solar cell structures incorporating blanket bsfsformed according to an embodiment of the invention;

FIG. 9A through FIG. 9I are cross-section view diagrams illustrating anexample process schematic for forming a solar cell incorporating anexample embodiment of the invention;

FIG. 10A through FIG. 10G are a mix of cross-section and plan-viewdiagrams illustrating an example process schematic for forming a HITcell incorporating an example embodiment of the invention;

FIG. 11 is a flow diagram illustrating techniques for forming at leastone doped, conductive crystalline region on a surface of a crystallinesemiconductor layer, according to an embodiment of the presentinvention; and

FIG. 12 is a flow diagram illustrating techniques for forming a doped,conductive crystalline region on a surface of a crystallinesemiconductor layer, according to an embodiment of the presentinvention.

DETAILED DESCRIPTION OF EMBODIMENTS

As described herein, an aspect of the present invention includes laserdoping of crystalline semiconductors. At least one embodiment of theinvention includes creating heavily-doped, conductive crystallineregions in selected areas of a surface of a crystalline basesemiconductor layer by locally laser-melting a dopant-source overlayerstack containing doped amorphous silicon (for example, doped-aSiH in ani-aSiH(bottom)/doped-aSiH(top) bi-layer).

As further detailed herein, the constituents of an i-aSiH/doped-aSiHstack in laser-irradiated regions (dopants plus matrix in which thedopants are contained) can largely remain in the structure after beingconverted to crystalline form. Additionally, the laser-melted regionscan be blanket or patterned (dots, lines, etc.). For the case ofpatterned features, the doped amorphous silicon stack remains in thestructure (between the laser irradiated regions) as a passivant. Also,in at least one embodiment of the invention, the laser melting cansimultaneously pattern (form openings in) various other layers (forexample, dielectric layers) over the dopant-containing layers.

Further, the techniques detailed herein can be used and/or implementedin connection with any structure (particularly silicon solar cellstructures) in which heavily doped crystalline layers are needed.Additionally, such techniques can be implemented in cases where thereexists a need or desire to avoid high temperature (for example, greaterthan 250-400 C) processing.

As also described herein, at least one embodiment of the inventionincludes a structure (for example, a solar cell structure) containing acrystalline semiconductor having a surface in which heavily dopedcrystalline regions are disposed in a surrounding surface region of arelatively lightly-doped semiconductor, wherein the lightly-dopedsemiconductor in the field region is passivated with an amorphoussilicon layer stack containing the same dopant as present in the heavilydoped region.

As noted below, at least one embodiment of the invention includes laserdoping using passivating i-aSiH(bottom)/B-doped p-aSiH(top) bi-layerstacks as dopant sources, wherein the i-aSiH layer passivates the Sisurface and the B-doped p-aSiH layer provides a source of B dopant tothe irradiated area.

FIG. 1A through FIG. 1C are cross-section view diagrams illustrating anexample process schematic for using a dopant-containing amorphoussilicon layer stack to form localized laser doped crystalline regions.FIG. 1A depicts dopant-containing amorphous silicon layer stack 100which includes an amorphous i-aSiH layer 102 and a B-doped p-aSiH layer104 disposed on an underlying Si substrate 106 that is irradiated bypatterned radiation 108. Patterned radiation 108 is typically providedby one or more pulses of a pulsed laser (pulse length <100 nanoseconds(ns)) so as to reduce thermal diffusion and bulk substrate heating. FIG.1B shows locally melted region 110 (which includes the irradiatedportion of stack 100 as well as an upper portion of substrate 106 belowit) produced by patterned radiation 108. While region 110 is melted, Bdopant diffuses from the source layer 102 portion of the melt into thesubstrate 106 portion of the melt. FIG. 1C shows the melted/diffusedirradiated region 110 after crystallization into laser-doped crystallineregion 112, where the crystallization has been templated by theunderlying crystalline substrate 106. Also, the i-aSiH/B-doped p-aSiH(102/104) layer stack remains as a passivant in the non-irradiatedregions.

The laser doping process illustrated in FIG. 1A through FIG. 1C canalternatively be implemented with one or more overlayers disposed on thedopant source layer stack. Such overlayers might include antireflectioncoatings (ARCs), as well as other dielectric and/or transparentreflector or barrier layers (such as SiO₂ and/or SiN).

FIG. 2A through FIG. 2C are cross-section view diagrams illustrating anexample process schematic with a dielectric overlayer coating or layerstack 230, according to an embodiment of the invention. In this case,the patterned laser irradiation 108 that induces the i-aSiH/B-dopedp-aSiH (102/104) melting also opens the overlayer to form openings 232in patterned overlayer 230′, as illustrated in FIG. 2B. FIG. 2C showsmelted/diffused irradiated region 110 after crystallization intolaser-doped crystalline region 112, where the crystallization has beentemplated by the underlying crystalline substrate 106. As before, thei-aSiH/B-doped p-aSiH (102/104) layer remains as a passivant in thenon-irradiated regions. It should be noted that the same overlayer stack230 might function both as an ARC during laser processing (where it canproduce an increase in the absorption of the laser) and areflection-enhancing layer when disposed in a completed solar cell, forexample, between a Si substrate back surface and Al back contact layer.

FIG. 3A and FIG. 3B as well as FIG. 4A and FIG. 4B are cross-sectionview diagrams illustrating two additional configurations of overlayersthat may be used in combination with a dopant-containing amorphoussilicon layer stack to form localized laser-doped crystalline regions,according to an embodiment of the invention. In a first additionalconfiguration, a metallic overlayer 300 (for example, Al) is disposed ona structure (such as depicted in FIG. 1A), as shown in FIG. 3A.Patterned irradiation 108 then forms a melted region (such as describedin connection with FIG. 1 and/or FIG. 2) which crystallizes intolocalized doped crystalline region 312, as shown in FIG. 3B, where thecrystallization has been templated by the underlying crystallinesubstrate 106. Metallic overlayer 300 is typically removed in theirradiated area, but reflowed at edges of the irradiated area to makereflowed edge contacts 320 with the laser doped crystalline region 312.

In a second additional configuration, a metallic overlayer 300 (forexample, Al) is disposed on the structure (such as depicted in FIG. 2A),as shown in FIG. 4A. Patterned irradiation 108 then forms a meltedregion (as detailed above), which crystallizes into localized dopedcrystalline region 312, as shown in FIG. 4B, where the crystallizationhas again been templated by the underlying crystalline substrate 106.Also, the patterned laser irradiation 108 that induces thei-aSiH/B-doped p-aSiH (102/104) melting opens/partitions overlayer 230to form openings 232 and patterned overlayer 230′. As noted in FIG. 3Babove, metallic overlayer 300 is typically removed in the irradiatedarea, but reflowed at edges of the irradiated area to make reflowed edgecontacts 320 with the laser doped crystalline region 312.

The presence of metallic overlayers in the stack before laser processingcan reduce peripheral heating/collateral damage to the dopant-containingamorphous silicon stack 102/104 at the edges of the irradiated area forcases in which the patterned laser radiation has a spatially non-uniformintensity or fluence profile that is high at the center and low at theedges. This can occur because the Al is only opened in the high fluencecenter portion of the irradiated region, resulting in an aperture ormask for transmission into the substrate that is smaller than the laserspot dimensions. Low fluence radiation outside of the center region canbe efficiently reflected by the remaining Al, thus reducing heat-induceddepassivation effects at the spot edges. While incorporating Al into thestack may include an extra step for certain applications (and thus acost adder), it would not necessarily include an extra step in processflows in which an Al deposition step after laser processing can bereplaced by one before laser processing.

It should be appreciated by one skilled in the art that while FIGS. 1through 5 depict the case of i-aSiH/doped-aSiH stack containing B,implementation of the invention with other passivating amorphous stacksis also contemplated. For example, the i-aSiH/doped-aSiH stack mayinclude alternative dopants (of either the same or opposite doping type;for example, phosphorous (P) being an example of an opposite dopingtype), as well as carbon (C) and/or germanium (Ge) replacing some or allof the Si in the aSiH and doped-aSiH layers, as well as additional dopedand undoped aSiH layers. In addition, it should also be noted that thematerials of the amorphous i-aSiH/doped-aSiH stack can include embeddednanocrystalline or microcrystalline semiconductor regions, that is, theterm “amorphous” should be taken to include the purely amorphous phaseas well as the amorphous phase with embedded nanocrystalline ormicrocrystalline regions. Likewise, while FIGS. 1 through 5 depict thecase of a crystalline silicon substrate, the substrate may include orcomprise other semiconductors (and/or layers of other semiconductors)such as Ge and/or SiGe alloys.

FIG. 5A through FIG. 5G include cross-section view diagrams illustratingexamples of different solar cell structures in which the localized laserdoped crystalline regions may be incorporated as localized bsfs, inaccordance with at least one embodiment of the invention. The solarcells of FIG. 5A through FIG. 5G are formed with p-type Si substrates406, and have a generic front structure 410 that includes selectiveemitter with lightly doped n-type regions 422 and heavily doped n-typeregions 424. The depicted solar cells additionally include ARC 426 and afront conductive finger/bus grid 428. As would be appreciated by oneskilled in the art, alternative front structures may also be employed.

Further, in FIG. 5A through FIG. 5G, back structures 450 include heavilydoped p-type crystalline localized laser bsf regions 412 (typically in apattern of separated dots, but alternatively a pattern of grid lines),dopant-containing amorphous silicon layer stack 460 that also provides apassivation function, and a metallic back contact layer (for example,Al). In the structure of FIG. 5A, the metallic back contact layer 464 isblanket and directly on both the local bsf 412 and amorphous siliconstack 460.

The structure of FIG. 5B is similar to the structure of FIG. 5A, butincludes a thin barrier layer 470 (for example, 15 nanometers (nm) ofSiN or Al₂O₃ deposited by plasma-enhanced chemical vapor deposition(PECVD)) patterned with contact openings under blanket metallic backcontact layer 464 to prevent contact layer/amorphous stack reaction.

As would be appreciated by one skilled in the art, internal opticalreflectivity at the Si side of a Si/Al interface can be significantlyincreased by inserting a transparent layer of a suitably selectedrefractive index and thickness between the Si and the Al; a highinternal reflectivity is desirable for high efficiency cells becausephotons not absorbed during a first pass through the cell substrate havea second chance to be absorbed when reflected back into the cell. Thestructure of FIG. 5C is similar to that of FIG. 5B, but with thesubstitution of patterned (typically dielectric) back layer stack 472having a thickness suitable for functioning as a reflector layer as wellas a barrier (for example, SiN (15 nm)/SiO₂ (90 nm) or SiO₂ (110 nm)alone). The structure of FIG. 5D utilizes a blanket transparentconductive reflector layer 474 (for example, 80 nm of Al-doped ZnO orSnO₂-containing In₂O₃) under blanket metallic back contact layer 464.This structure has the advantage of providing a barrier/reflectorfunction without the need for contact open patterning. The structure ofFIG. 5E is similar to that of FIG. 5D except blanket metallic conductorlayer 464 is replaced by grid-patterned metallic contact layer 476. Thestructure of FIG. 5F is similar to the structure of FIG. 5C, except thatblanket metallic contact layer 464 is replaced by patterned metalliccontact layer 478 with opening 480 above localized bsf 412 and reflowededge contacts 482. The structure of FIG. 5G is identical to that of FIG.5F, except that it further includes both back contact layer 478 and backcontact layer 464.

The processes of FIGS. 1-4 may also be used to form n-type localizedbsfs in n-type Si substrates if p-type dopant-containing source layerstack 102/104 is replaced with an n-type dopant-containing source layerstack. Likewise, solar cell structures analogous to those shown in FIG.5A through FIG. 5G may alternatively be fabricated with n-type Sisubstrates. In such an instance, p-type dopant-containing source layerstack 102/104 would be replaced with an n-type dopant-containing sourcelayer stack, p-type localized laser bsf 412 would be replaced with ann-type localized laser bsf, and n-type emitter 440/442 would be replacedwith a p-type emitter.

While FIGS. 1 through 5 show patterned (or localized) embodiments of theinvention, it is noted herein that aspects of the invention may beimplemented in large-area (for example, blanket) modes as well. Forinstance, a blanket laser BSF embodiment of the invention can beimplemented on a Si substrate by irradiating blanket regions of adopant-containing amorphous Si layer stack disposed on a Si substratehaving the same dopant type. The irradiation can be performed withlarge-area (for example, 1×1 square centimeters (cm²)) laser spots (asshown in FIG. 6) or in a rastered mode in which a small-area spot (forexample, 50 μm diameter, though line shaped spots are also possible) isscanned so as to completely cover the desired area (as shown in FIG. 7).

Specifically, FIG. 6A depicts, in cross-section view a startingstructure for implementation of a blanket laser bsf on p-type substrate106 using a p-doped aSiH stack (104/102). The structure of FIG. 6A isirradiated with large-area irradiation 508, resulting in dopedconductive crystalline region 512, as shown in FIG. 6B. Large-areairradiation 508 is typically provided by one or more pulses of a pulsedlaser (pulse length <100 ns) so as to reduce thermal diffusion and bulksubstrate heating. Substrates larger than the lateral dimensions ofirradiation 508 may be irradiated, for example, in a step and repeatmode. In a typical example of step and repeat irradiation, the step sizewould be comparable to the lateral dimensions of irradiated region.

Accordingly, FIG. 7A through FIG. 7E include cross-section view diagramsillustrating an example process schematic for fabricating blanket laserback surface field in a rastered mode. Specifically, FIG. 7A depicts astarting structure for implementation of a blanket laser bsf on p-typesubstrate 106 using a p-aSiH stack (102/104). The structure of FIG. 7Ais irradiated with spot irradiation 608, resulting in doped conductivecrystalline region 612, as shown in FIG. 7B. FIG. 7B through FIG. 7Dshow successive stages of the process as spot irradiation 608 is scannedor rastered to enlarge the size of region 612 to its final size shown inFIG. 7E. Spot irradiation 608 is typically provided by a pulsed laser(pulse length <100 ns) so as to reduce thermal diffusion and bulksubstrate heating. It is noted that the irradiated regions provided bysequential pulses of spot irradiation 608 would typically havesubstantial overlap, and that such rastered irradiation may be viewed asa form of step and repeat irradiation implemented with a step sizesignificantly smaller than the spot size.

FIG. 8A through FIG. 8C include cross-section view diagrams illustratingexamples of three different solar cell structures in which thelarge-area and/or blanket crystalline doped regions of the invention maybe incorporated as blanket bsfs. Similar to the solar cells depicted inFIG. 5A through FIG. 5G, the solar cells of FIG. 8A through FIG. 8C areformed with p-type Si substrates 406, and have a generic front structurethat includes a selective emitter with lightly doped n-type regions 422and heavily doped n-type regions 424. The solar cells of FIG. 8A throughFIG. 8C also include front ARC 426 and a finger/bus grid 428.

The structure of FIG. 8A has a blanket bsf 712 with a blanket metallicback contact layer 720 (for example, Al). The structure of FIG. 8Bfurther includes a blanket reflector layer in the form of a transparentconductor layer 736 between the blanket bsf 712 and metallic backcontact layer 720. The structure of FIG. 8C is similar to the structureof FIG. 8B, except for the substitution of a via-patterned (orline-patterned) transparent reflector layer 738 for blanket reflectorlayer 736. Patterning of transparent reflector layer 738 may beaccomplished, for example, by pulsed laser processing similar to thatshown in FIG. 2, or by conventional lithography and wet etching.Reflector layer 738, which typically includes one or more transparentdielectric layers, and reflector layer 736 have several functions. Inaddition to the reflection-enhancing function mentioned in connectionwith the FIG. 4 layers 472 and 474, reflector layers 736 and 738 canalso reduce the back surface minority carrier recombination velocitybecause recombination at bsf/metal interface 712/720 is likely to beworse than at bsf/reflector interfaces 712/736 and 712/738.

Solar cell structures analogous to those shown in FIG. 8A through FIG.8C may alternatively be fabricated with n-type Si substrates instead ofthe p-type Si substrates shown. In such instances, p-type blanket laserbsf 712 would be replaced with an n-type blanket laser bsf, and n-typeemitter 422/424 would be replaced with a p-type emitter.

FIG. 9A through FIG. 9I include cross-section view diagrams illustratingan example process schematic for a process flow incorporating an exampleembodiment of the invention. Specifically, FIG. 9A through FIG. 9Idepict an example process flow and structure for a cell that has ageneric laser-doped selective emitter on the cell front and a localizedlaser bsf (produced via at least one embodiment of the invention) on thecell back. It should be noted and appreciated that the order of certainsteps is not critical, and that other steps, shown or not shown, may beadded, combined, substituted with other steps producing comparablefunction, or eliminated, depending on the final structure desired, aswould be known to those skilled in the art.

FIG. 9A through FIG. 9F depicts an initial portion of an example processflow, namely the steps for forming a solar cell front. FIG. 9A depictsselecting a p-type c-Si substrate 106. FIG. 9B depicts performing aPOCl₃ diffusion to form a lightly-doped n-type emitter layers 422 and422′ on both sides of substrate 106. FIG. 9C depicts forming passivatingARC layer stack 426 (a layer of PECVD SiN, for example). FIG. 9D depictsremoving the back surface emitter layer 422′ (by a process such asetching in a solution of tetramethylammonium hydroxide (TMAH)). FIG. 9Edepicts forming heavily doped selective emitter 424 and finger/gridopenings 427 in passivating ARC layer stack 426, which may beaccomplished by forming a n-type (for example, phosphorus) doping sourcelayer (for example, a phosphosilicate glass (PSG), not shown) on ARClayer stack 426, irradiating the structure in selected regions with apatterned pulsed laser radiation to both locally diffuse the dopant andform finger/grid openings in the ARC layer stack, and then removingunreacted portions of the doping source layer. Further, FIG. 9F showsthe structure of FIG. 9E after deposition of conductive front grid/busstructure 428.

FIG. 9G through FIG. 9I show the steps of the example process flowparticular to an embodiment of the invention, namely the steps that maybe used to form a solar cell back that includes laser doped crystallinelocal bsf regions. FIG. 9G depicts forming a back surface p-typedopant-containing aSiH/reflector-barrier stack comprising i-aSiH layer102, B-doped p-aSiH layer 104, and overlayer stack 830. Overlayer stack830 is typically a transparent reflector/barrier layer 830 and mayinclude, for example, a lower layer of PECVD SiN (15 nm) in contact withdoped layer 104 and an upper layer of PECVD SiO₂ (100 nm).

Patterned laser irradiation is applied to the back of structure of FIG.9G to form patterned overlayer stack 830′ with openings 832 over heavilyp-type doped localized bsf regions 850. The irradiation can, forexample, be in a pattern of spaced-apart dots (for example, 50 μmdiameter dots on a pitch of 1 millimeter mm), but can also be applied toform a pattern of spaced-apart lines. Additionally, a metallic backcontact layer 870 (for example, Al) is deposited over patternedoverlayer stack 830′ and lbsf regions 850 to make the completed solarcell structure of FIG. 9I.

Exemplary fabrication conditions and materials characteristics of thelaser doped regions of FIG. 1 through FIG. 5 will now be described inadditional detail for two specific examples: (i) a rastered dotirradiation to form blanket laser bsf regions (such as might beperformed on the structure of FIG. 6A), and (ii) a step and repeat dotirradiation to form localized laser bsf regions and overlayer patterning(such as might be performed on the structure of FIG. 2A). Both examplescan utilize the same dopant-containing amorphous silicon layer stack102/104 that includes i-aSiH layer 102 and B-doped p-aSiH layer 104,deposited at 250 C by PECVD using 13.56 MHz plasma excitation. Thei-aSiH layer can be deposited by PECVD on an HF-last cleaned Si surfacefrom a 1:10 mixture of SiH₄ and H₂ at a pressure of 8 Torr and theB-doped p-aSiH layer deposited on the i-aSiH from a 1:3.4:11 mixture ofSiH₄, B₂H₆, and H₂ at a pressure of 4 Torr. The thickness of the i-aSiHlayer can be, in this example, fixed at 10 nm whereas the thickness ofthe B-doped p-aSiH layer can be in a thickness range of 10 to 100 nm,with 20-50 nm being preferable.

For example (ii), the overlayer stacks can include one or more layers ofPECVD SiO₂ and/or SiN, deposited at 250-400 C from mixtures of SiH₄/N₂Oor SiH₄/N₂, typically to a combined thickness in the range 80 to 110 nm.These layer stacks provide excellent passivation, with minority carrierlifetimes of 2-5 milliseconds (ms) measured (by microwavephotoconductance) for p-type Czochralski-grown (CZ) Si wafers (surfaceorientation 100, resistivity 18 ohm-cm, thickness 720 μm) coated on bothsides.

Additionally, patterned irradiation can be primarily provided by a diodepumped Q-switched laser providing irradiation to a ˜40-50 μm diameterspot in 20-30 ns pulses at a repetition rate of 50-60 kHz, averagepowers of 4-13 W, and wavelengths of 532 or 1064 nm. By way of example,the laser spot position can be fixed and the sample mounted on atranslation stage. For rastered samples (used to produce blanket bsfs),a sample stage can be scanned at 10 cm/sec in a back-and-forth patternunder the laser spot to draw parallel lines spaced apart by 40 μm, aprocedure which provided lines of overlapping spots with acenter-to-center spacing equal to the scan rate divided by the laserrepetition rate (for example, 2 μm center-to-center spacing for a scanrate of 10 cm/sec and a repetition rate of 50 kHz). Samples to bepatterned with spaced-apart dots (for localized bsfs) can be exposed ina step-and-repeat mode in which the sample is stationary while beingexposed to a selected number of pulses (typically 5 to 20) before beingmoved for irradiation at the next spot location (typically 1 mm away).Example conditions can include wavelength 532 nm, 5-6 W average power,repetition rate 60 kHz, and scan rate 10 cm/sec for the scanned samples,and N=5 for the step-and-repeat samples.

Sheet resistance (Rs) measurements (4-point probe, after correction forsubstrate conductivity) for these rastered conditions on the CZ Siwafers described above can show, for example, approximately 9 ohm/sq fora 50 nm B-doped p-aSiH film, with the expected inverse Rs scaling withp-aSiH film thickness (for example, 25 ohm/sq for a p-aSiH filmthickness of 20 nm). In one example, the Rs values were unaffected bythe presence of the SiN and/or SiO₂ overlayers used. Secondary ion massspectrometry (SIMS) analysis indicated the B concentration to be6e21/cm³ in the as-deposited p-aSiH films. Also, after irradiation, thedopant profile was approximately box-like, with a depth of 0.5 μm and anaverage concentration of around 5e20/cm³.

While some above-described embodiments of the invention have focused onphotovoltaic (PV) applications such as blanket laser BSF and localizedlaser BSF (dot pattern, lines, grids, etc.), blanket laser emitter andselective laser emitter embodiments of the invention (on either thefront or back surface of a solar cell) are possible as well. By way ofexample, a selective laser emitter embodiment of the invention can beimplemented on p-type substrates by forming a blanket crystalline n-typelayer by POCl₃ diffusion, depositing a passivating n-typedopant-containing amorphous silicon layer stack and ARC overlayer on theblanket crystalline n-type layer, and patterning the amorphous siliconlayer stack and ARC overlayer to form a heavily-doped crystallineselective emitter in a finger/grid pattern under openings in the ARClayer. Analogous embodiments may be implemented in n-type substrateswith the substitution of layers with opposite doping types.

While the techniques described and illustrated above have been appliedto standard homojunction crystalline Si solar cells (that is, non-HITcells), embodiments of the invention can also be implemented in HITcells as well. As described, for example, by M. Taguchi et al. in “HIT™cells—high efficiency crystalline Si cells with novel structure” [Prog.Photovolt: Res. Appl. 8: 503-513 (2000)], HIT cells typically include ani-aSiH/doped-aSiH stack of one doping type on one side of asemiconductor substrate to function as a blanket emitter and ani-aSiH/doped-aSiH stack of opposite doping type on the opposite side ofthe substrate to function as a blanket back surface field. Both dopantstacks are coated with a transparent conductor and metallic finger/busgrid. As noted above, a challenge with HIT cells is the narrow processwindow for providing i-aSiH/doped-aSiH stacks with aSiH layers thickenough to provide good passivation yet thin enough to provide sufficienttunneling current to the back surface metallurgy.

In an embodiment of the invention, localized crystalline regions of highdopant concentration (and conductivity) may be created from thei-aSiH/doped-aSiH layer stacks already in the cell structure. Theselocalized regions provide a parallel, high-conductivity path to the backsurface metallurgy (typically, as noted above, a blanket layer oftransparent conductor on which is disposed a metallic finger/gridpattern). In particular, such localized regions may be incorporated intothe emitter side of a conventional HIT cell, the bsf side of aconventional HIT cell, or both the emitter and bsf sides of aconventional HIT cell. These localized regions may also be incorporatedinto the HIT side of hybrid HIT cells in which there is a HIT structureon one side of the cell and a conventional structure (for example thecell front structure of FIG. 5A through FIG. 5G) on the other side ofthe cell.

FIG. 10A through FIG. 10G include a mix of cross-section and plan-viewdiagrams illustrating an example process schematic for the particularcase of forming a HIT-type solar cell incorporating the laser-dopedcrystalline regions of the invention as part of a bsf. Specifically,FIG. 10A depicts in cross-section view a starting p-type Si substrate900. FIG. 10B depicts the structure of FIG. 10A after deposition of ani-aSiH/n-aSiH amorphous silicon stack 902/904 on an upper surface ofp-type substrate 900 and an i-aSiH/p-aSiH stack 906/908 on a lowersurface of p-type substrate 900, where the thicknesses of i-aSiH anddoped aSiH layers are typically in the range 2-20 nm. FIG. 10C depictsthe structure of FIG. 10B after laser patterned laser irradiation toform localized heavily doped crystalline p-type regions 912 on a backsurface of substrate 900 (which may be a pattern of dots or grid lines,as indicated in the bottom plan view FIG. 10F and FIG. 10G).

FIG. 10D depicts the structure of FIG. 10C after application of back andfront transparent conductor layers 974 and 974′. The completed cellstructure, formed by depositing back and front metallic grids 976 and976′ on the structure of FIG. 10D, is shown in FIG. 10E. It should bereadily apparent to those skilled in the art how the process flows ofFIG. 9A through FIG. 9I and FIG. 10A through FIG. 10E may be modifiedfor fabricating any of the alternative HIT cell embodiments mentionedherein.

FIG. 11 is a flow diagram illustrating techniques for forming at leastone doped, conductive crystalline region on a surface of a crystallinesemiconductor layer (for example, a localized laser back surface fieldregion of a solar cell), according to an embodiment of the presentinvention. Step 1102 includes forming a dopant-containing amorphoussilicon layer stack (for example, a lower layer of undoped amorphoussilicon providing a passivating function and an upper layer of dopedamorphous silicon providing a dopant source function) on at least oneportion of a surface of a crystalline semiconductor layer. The amorphoussilicon layer stack, as detailed herein, includes a purely amorphousphase as well as an amorphous phase with embedded nanocrystalline and/ormicrocrystalline regions.

The forming step can include, for example, forming ani-aSiH(bottom)/p-aSiH(top) bi-layer on the portion of the surface of thecrystalline base semiconductor layer, where the undoped (intrinsic)i-aSiH passivating layer may be 10 nm thick and the p-type dopant sourcelayer p-aSiH may be a B-doped aSiH layer that is 10 to 50 nm thick.Also, in another embodiment of the invention the B-doped aSiH layer canhave a thickness of approximately 2-20 nanometers i-aSiH+10-50 aSiHB.Also, at least one embodiment of the invention can include forming atransparent layer or layer stack on the aSiH(bottom)/doped-aSiH(top)layer, as well as forming a conductive contact layer on the transparentlayer stack. Further, at least one embodiment of the invention includesirradiating at least one selected area of the dopant-containingamorphous silicon layer stack to form at least one localized crystallineregion of high doping concentration over which the transparent layerstack has been removed.

Additionally, the dopant-containing amorphous silicon layer stack caninclude a layer of undoped amorphous silicon providing a passivatingfunction and a layer of doped amorphous silicon providing a dopantsource function. The layer of undoped amorphous silicon and/or the layerof doped amorphous silicon can include a-SiH, a-Si(Ge)H, a-Ge(Si)H,and/or a-GeH, wherein H content varies from approximately 5% toapproximately 50% atomic percent. Also, the layer of undoped amorphoussilicon and/or the layer of doped amorphous silicon can additionallyinclude carbon (C) and/or one or more dopants selected from a groupincluding boron (B), phosphorous (P), arsenic (As), antimony (Sb),nitrogen (N), gallium (Ga), indium (In), and aluminum (Al).

Step 1104 includes irradiating at least one selected area of thedopant-containing amorphous silicon layer stack, wherein thedopant-containing amorphous silicon layer stack interacts with an upperportion of the underlying semiconductor layer to form a doped,conductive crystalline region in the at least one irradiated area. Theirradiating step can include providing laser radiation to locally heatat least one selected area of the layer stack and one or more underlyingbase semiconductor layer regions. Additionally, the irradiating step caninclude irradiating at least one selected area of the silicon layerstack and one or more underlying base semiconductor layer regions toform at least one localized region of high doping concentration as wellas a self-aligned opening in the overlayer stack above said localizedcrystalline region.

Additionally, the overlayer stack can include at least one metalliclayer that is reflowed over one or more edges of said self-alignedopening to make contact with said localized crystalline region. Further,at least one embodiment of the invention includes forming a conductivecontact layer over the overlayer stack and exposed localized crystallineregions remaining after irradiation.

The irradiating step can also include irradiating a pattern selectedfrom a group including a blanket pattern, a grid pattern, a fingerand/or bus pattern, and a spaced-apart dots pattern. The irradiatingstep is preferably provided by one or more pulses of a pulsed laser(pulse length <100 ns) so as to reduce thermal diffusion and bulksubstrate heating.

The techniques depicted in FIG. 11 can also include forming a blankettransparent conductor layer over the dopant-containing amorphous siliconlayer stack and localized crystalline regions remaining afterirradiation, and forming a metallic conductor layer on the transparentconductor layer, wherein the metallic conductor has a grid pattern or ablanket pattern.

As detailed herein, the dopant-containing amorphous silicon layer stackremains intact in non-irradiated areas. The techniques depicted in FIG.11 can additionally include selecting a p-type Si substrate. Further, atleast one embodiment of the invention includes forming an overlayerabove the dopant-containing amorphous silicon layer stack formed in step1102. The overlayer can include a single or multilayer ARC or dielectriccoating, a back reflector, a diffusion barrier, and/or a transparentconductor. After the irradiation of step 1104, the overlayer wouldinclude one or more openings self-aligned to the at least one doped,conductive crystalline region formed via said irradiating step.

Additionally, as described herein, at least one embodiment of theinvention includes a structure that includes a crystalline semiconductorhaving at least one surface, a doped crystalline region disposed in atleast one selected area of the semiconductor surface, and adopant-containing amorphous silicon layer stack containing a same dopantas present in the doped crystalline region on at least a portion of thesemiconductor surface outside the selected area, wherein thedopant-containing amorphous silicon layer stack passivates the portionof the semiconductor surface on which it is disposed. The structure canadditionally include an overlayer formed above the silicon layer stack,wherein the overlayer includes a single or multilayer antireflectioncoating (ARC), a back reflector, a diffusion barrier, and/or atransparent conductor.

As detailed herein, such a structure can include a blanket layer of ametallic conductor disposed on the dopant-containing amorphous siliconlayer stack and the doped crystalline region. Accordingly, such astructure can be one face of a solar cell structure. Additionally, atleast one embodiment of the invention can also include a blanket layerof a transparent conductor disposed on the dopant-containing amorphoussilicon layer stack and the doped crystalline region, and a metallicconductor layer disposed on the blanket layer of transparent conductorwherein the metallic conductor layer has a grid pattern or a blanketpattern. This particular structure can also be one face of a solar cellstructure.

Further, a structure of at least one embodiment of the invention caninclude a patterned overlayer stack disposed on the dopant-containingamorphous silicon layer stack, wherein the patterned overlayer stack ispatterned with at least one opening over the doped crystalline region,and a metallic conductor layer disposed over the patterned overlayerstack and doped crystalline region. The overlayer stack can include atleast one of a single or multilayer dielectric coating, a backreflector, a diffusion barrier, a transparent conductor, and a metallicconductor. Such as structure can also be one face of a solar cellstructure.

In such a structure, the dopant-containing amorphous silicon layer stackcan include a layer of undoped amorphous silicon providing a passivatingfunction and a layer of doped amorphous silicon providing a dopantsource function. The layer of undoped amorphous silicon and/or the layerof doped amorphous silicon can include, as noted herein, one of a-SiH,a-Si(Ge)H, a-Ge(Si)H, and a-GeH, wherein H content varies fromapproximately 5 to approximately 50 atomic percent. Further, the layerof undoped amorphous silicon and/or the layer of doped amorphous siliconin such a structure can also include C and/or one or more dopantsselected from a group including B, P, As, Sb, N, Ga, In, and Al.

Also, in at least one embodiment of the invention, the structure caninclude a conductive overlayer formed over the structure, and thestructure can be one face of a solar cell structure.

It is also to be appreciated that all or portions of at least oneembodiment of the present invention may be implemented in a wide varietyof PV and non-PV devices, PV device geometries (including interdigitatedback contact geometries, bifacial cell geometries, front surfacefield/emitter-on-the-back geometries, etc.), and PV fabrication schemes.

The resulting PV and non-PV devices may be distributed by the fabricatoras single cells or devices in raw form, as single cells or devices withpackaging, or as single cells or devices in a multichip package that mayinclude PV devices and components functionalities other than PV.

FIG. 12 is a flow diagram illustrating techniques for forming a doped,conductive crystalline region on a surface of a crystallinesemiconductor layer (for example, a laser back surface field region of asolar cell), according to an embodiment of the present invention. Step1202 includes forming a dopant-containing amorphous silicon layer stackon at least one portion of a surface of a crystalline semiconductorlayer. The techniques depicted in FIG. 12 can additionally includeselecting a p-type Si substrate.

Step 1204 includes irradiating the dopant-containing amorphous siliconlayer stack, wherein the dopant-containing amorphous silicon layer stackinteracts with the underlying crystalline semiconductor layer to form ablanket doped, conductive crystalline region. The irradiating step ispreferably provided by one or more pulses of a pulsed laser (pulselength <100 ns) so as to heat the dopant-containing amorphous siliconlayer stack with a minimum of bulk substrate heating.

Although illustrative embodiments of the present invention have beendescribed herein with reference to the accompanying drawings, it is tobe understood that the invention is not limited to those preciseembodiments, and that various other changes and modifications may bemade by one skilled in the art without departing from the scope orspirit of the invention. The terminology used herein was chosen to bestexplain the principles of the embodiments, the practical application ortechnical improvement over technologies found in the marketplace, or toenable others of ordinary skill in the art to understand the embodimentsdisclosed herein.

What is claimed is:
 1. A method for forming at least one doped,conductive crystalline region on a surface of a crystallinesemiconductor layer, the method comprising: forming a dopant-containingamorphous silicon layer stack on at least one portion of a surface of acrystalline semiconductor layer; and irradiating a selected area of thedopant-containing amorphous silicon layer stack, wherein the selectedarea of the dopant-containing amorphous silicon layer stack interactswith an upper portion of the underlying crystalline semiconductor layerto form a doped, conductive crystalline region, and each non-selectedarea of the dopant-containing amorphous silicon layer stack remainsintact on the at least one portion of the surface of the crystallinesemiconductor layer.
 2. The method of claim 1, wherein said irradiatingcomprises providing pulsed laser radiation to locally heat the selectedarea of the layer stack and an upper portion of the semiconductor layerunderlying the selected area of the layer stack.
 3. The method of claim1, wherein said forming a dopant-containing amorphous silicon layerstack comprises forming an i-aSiH(bottom)/doped-aSiH(top) layer on thesurface of the crystalline semiconductor layer.
 4. The method of claim3, wherein the dopant-containing amorphous silicon layer stack comprisesa purely amorphous phase as well as an amorphous phase with embeddednanocrystalline and/or microcrystalline regions.
 5. The method of claim1, wherein said forming a dopant-containing amorphous silicon layerstack further comprises: forming an overlayer stack on thedopant-containing amorphous silicon layer stack, wherein the overlayerstack comprises at least one of a single or multilayer dielectriccoating, a back reflector, a diffusion barrier, a transparent conductor,and a metallic conductor, and wherein said irradiating comprisesirradiating at least one selected area of the dopant-containingamorphous silicon layer stack to form at least one localized crystallineregion of high doping concentration as well as a self-aligned opening inthe overlayer stack above said localized crystalline region.
 6. Themethod of claim 5, comprising: forming a conductive contact layer overthe overlayer stack and exposed localized crystalline regions remainingafter irradiation.
 7. The method of claim 1, comprising: forming ablanket transparent conductor layer over the dopant-containing amorphoussilicon layer stack and localized crystalline regions remaining afterirradiation.
 8. The method of claim 7, comprising: forming a metallicconductor layer on the blanket transparent conductor layer, wherein themetallic conductor has a grid pattern or a blanket pattern.
 9. Themethod of claim 1, wherein the dopant-containing amorphous silicon layerstack comprises (i) a lower layer of undoped amorphous silicon providinga passivating function and (ii) an upper layer of doped amorphoussilicon providing a dopant source function.
 10. The method of claim 9,wherein the lower layer of undoped amorphous silicon and/or the upperlayer of doped amorphous silicon comprise one of a-SiH, a-Si(Ge)H,a-Ge(Si)H, and a-GeH.
 11. The method of claim 10, wherein H contentvaries from approximately 5% to approximately 50% atomic percent. 12.The method of claim 9, wherein the lower layer of undoped amorphoussilicon and/or the upper layer of doped amorphous silicon furthercomprise carbon (C) and/or a dopant selected from a group includingboron (B), phosphorous (P), arsenic (As), antimony (Sb), nitrogen (N),gallium (Ga), indium (In), and aluminum (Al).
 13. A method for forming adoped, conductive crystalline region on a surface of a crystallinesemiconductor layer, the method comprising: forming a dopant-containingamorphous silicon layer stack on at least one portion of a surface of acrystalline semiconductor layer; and irradiating the dopant-containingamorphous silicon layer stack, wherein the dopant-containing amorphoussilicon layer stack interacts with an upper portion of the underlyingcrystalline semiconductor layer to form a blanket doped, conductivecrystalline region.
 14. The method of claim 13, wherein said irradiatingcomprises providing pulsed laser radiation to heat the dopant-containingamorphous silicon layer stack and an upper portion of the semiconductorlayer regions underlying the irradiated layer stack.